1. Field of the Invention
The present invention relates in general to a fluid-filled cylindrical elastic mount capable of reducing the amount of transmission of vibrations, based on flows of a fluid contained therein. More particularly, the invention is concerned with a simplified structure of a fluid-filled cylindrical elastic mount capable of effectively damping input vibrations having relatively low frequencies, and exhibiting a sufficiently low dynamic spring constant with respect to input vibrations having high frequencies.
2. Discussion of the Prior Art
A cylindrical elastic mount such as a suspension bushing or engine mount used on a motor vehicle is known as a device for elastically or flexibly connecting two members in a vibration system in a vibration damping or isolating manner, or as a device for mounting one of the two members on the other member such that the one member is elastically or flexibly supported by the other member. Such a cylindrical elastic mount has an inner and an outer sleeve which are made of a metallic material and are disposed in radially spaced-apart, coaxial or non-coaxial relationship with each other. Between these inner and outer sleeves, there is interposed an elastic body such as a formed rubber mass, such that the two sleeves are elastically connected by the elastic body.
The elastic mount of the type indicated above is generally required to provide an excellent vibration damping effect with respect to low-frequency vibrations, and exhibit a sufficiently low dynamic spring rate or constant with respect to high-frequency vibrations. Conventionally, the elasticity or elastic deformation or displacement of the elastic body is primarily relied upon to meet these two requirements concerning the vibration damping and/or isolating characteristics. However, the conventional elastic mount suffers from considerable difficulty in meeting the requirements, particularly the requirement for providing a satisfactory effect of damping the low-frequency vibrations.
In view of the above drawback of the conventional cylindrical elastic mount, a so-called fluid-filled mount has been recently proposed, as disclosed in Publication Nos. 48-36151 and 52-16554 of examined Japanese Patent Applications. A fluid-filled elastic mount as disclosed in these publications has two fluid chambers which are partially defined by an elastic body and are spaced apart from each other in a radial direction of the mount in which vibrations are received. The two fluid chambers are held in fluid communication with each other through a suitable orifice passage, so that the input vibrations having relatively low frequencies may be effectively damped by resonance of a fluid flowing through the orifice passage. The fluid flow is induced due to elastic deformation of the elastic body upon application of the vibrations to the fluid-filled elastic mount.
In the fluid-filled elastic mount indicated above, the orifice passage for causing the fluid resonance is tuned so as to provide excellent damping characteristic for the vibrations having relatively low frequencies. However, the elastic mount also receives vibrations whose frequencies are higher than those of the excellently damped vibrations. When the frequency range of the vibrations received by the elastic mount is higher than the tuned frequency range of the orifice passage, substantially no flows of the fluid through the orifice passage occurs, or the orifice acts as if it were closed. In this case, therefore, the fluid-filled elastic mount exhibits a considerably high dynamic spring constant, and tends to undesirably suffer from deteriorated characteristics for isolating the high-frequency vibrations.
To overcome the above problem, it is proposed to suitably lower the dynamic spring constant of the fluid-filled elastic mount, by employing a mechanism which includes an oscillating plate for absorbing the pressure of the fluid when the vibrations having relatively high frequencies are applied to the elastic mount. Examples of fluid-filled vibration isolating devices incorporating such a mechanism are shown in laid-open Publication Nos. 53-5376 and 57-9340 of unexamined Japanese Patent Applications. However, the use of the fluid pressure absorbing mechanism indicated above for the fluid-filled cylindrical elastic mount necessarily complicates the structure of the mount, and results in an increase in the cost of manufacture of the mount, thereby making it difficult to adopt the mechanism from the economical point of view.